1. Field of the Invention
The present invention relates to fuel cell including an electrolyte electrode assembly interposed between a pair of separators. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art:
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises a pair of electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field is formed on a surface of the separator facing the cathode for supplying-the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between the adjacent separators such that a coolant flows along the separators.
In the cooling system, a coolant such as water or conventional cooling liquid for vehicles contains impurities such as ions, or metallic additives. The coolant itself is electrically conductive. Thus, electrical leakages to the earth or liquid may occur, and an ion exchanger is required for preventing the leakages. If an ordinary coolant is used, the ion exchanger may not work sufficiently. Electrical leakages to the earth or liquid may occur at the time of starting operation of the fuel cell.
Therefore, pure water or the like needs to be used as the coolant in combination with the ion exchanger for keeping electrical conductivity of the coolant below a certain level. The ion exchanger needs to be periodically replaced for preventing electrical leakages to the earth or liquid. The maintenance operation is laborious, and the cooling system is not economical.
A solution to the problem proposed by the applicant of the present invention is discussed in the U.S. patent application publication No. US2001/0046618 A1. The publication discloses a fuel cell stack which effectively prevents electrical leakages through the coolant with a simple structure for maintaining a desired power generation performance.
The fuel cell stack comprises a power-generating cell which has a joined unit including an electrolyte interposed between an anode electrode and a cathode electrode. The joined unit is interposed between separators so that fuel gas is supplied to the anode electrode, while oxygen-containing gas is supplied to the cathode electrode. Further, the fuel cell stack comprises a pair of collecting electrodes which are electrically connected to a predetermined number of the power-generating cells in an integrated manner, a cooling cell to which cooling medium for cooling the power-generating cell is supplied and which is interposed between the collecting electrodes, an insulating means for electrically insulating the cooling medium from the power-generating cell and the collecting electrodes, and a conducting means for electrically connecting the power-generating cells arranged with the cooling cell interposed therebetween to one another, or the power-generating cell and the collecting electrodes to one another.